Researchers developed a faster strain-temperature sensor that features 1 million sensing points over a single 10-kilometer of standard fiber. It could be useful for monitoring the integrity of large structures and for biomedical sensing. Credit: This work is a derivative from Vasco da Gama Bridge by Duncan Rawlinson used under CC BY-NC 2.0 and licensed by Alejandro Dominguez-Lopez, University of Alcala, under CC BY-NC 2.0.

Distributed sensors are increasingly used these days to regularly monitor the structural health of large structures such as bridges or dams. A newly developed fiber optic distributed sensor, with 1 million sensing points, will be able to detect structural problems in a much faster manner compared to those that are currently available.

With fiber-based sensors, it is possible to precisely detect erosion or cracking before a dam fails, for example,. arlier detection of a problem means it might be possible to prevent it from getting worse or could provide more time for evacuation.

Alejandro Dominguez-Lopez, University of Alcala

Fiber optic distributed sensors are suitable for monitoring infrastructure as they are capable of being used in rough environments and also in areas that do not have a nearby power supply. For example, if a single fiber is placed along the length of a bridge, detectable changes in the light traveling down the fiber will be brought about by changes in the structure at any of the sensing points along the optical fiber. Despite the growing popularity of fiber optic distributed sensors, these sensors are still used primarily to monitor for landslides along railroads and detect leaks in oil pipes.

In The Optical Society (OSA) journal Optics Letters, Dominguez-Lopez and his colleagues from UAH and the Swiss Federal Institute of Technology (EPFL) present a report on the first fiber optic distributed sensor capable of sensing strain and temperature variations from 1 million sensing points over a 10-kilometer optical fiber in less than 20 minutes. Strain, referring to a measure of deformation, determines how much mechanical stress is on a structure or an object.

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The new sensor is almost 4.5 times faster than earlier reported sensors with 1 million sensing points. Even though a magic number is not available, more sensing points highlight the need for fewer fiber-optic units in order to monitor an entire structure. This indeed simplifies the overall sensing scheme and could also bring about a reduction in costs.

“Because we have such a large density of sensing points — one per centimeter — our optimized sensor could also be used for monitoring in applications such as avionics and aerospace, where it’s important to know what is happening in every inch of a plane wing, for example,” said Dominguez-Lopez.

Improving sensor performance

Brillouin optical time domain analysis is an approach used by the new sensor. This approach requires pulsed and continuous wave laser signals to interact. The researchers studied that the conventional method of generating the continuous signal caused distortions in the system at higher laser powers. Changing the way the laser signal was generated will help prevent these problems, permitting them to increase the laser power and thus enhance the sensing performance.

The detrimental effects that we studied and corrected have been affecting the performance of commercially-available Brillouin optical time domain sensors for some time. If manufacturers incorporate our optimization into their sensors, it could improve performance, particularly in terms of acquisition speed.

Alejandro Dominguez-Lopez, University of Alcala

The researchers used this new approach to demonstrate that they could measure the temperature of a hot spot to within 3 oC from the end of a 10-km long fiber.

New applications ahead

The researchers are currently working to make the sensor even faster by exploring ways to further bring down the acquisition time. They also plan to increase the density of sensing points to more than one per centimeter, which could indeed enable the technology to expand into totally new areas such as biomedical applications.

It is also possible to potentially adapt the optical fibers for use in textiles, where the sensors could help to screen for disease or monitor a person’s health. For instance, the researchers assume that it might possible to detect temperature deviations that are present in breast cancer by using the fiber optic sensors. For this type of application, it is essential to use more sensing points in a smaller area instead of using a particularly long fiber.

In our paper, we not only identified a major limitation of this sensing technique but also demonstrated a way to overcome that constraint. The new sensor could enable improved structural monitoring and help move this sensing technology into exciting new research areas and applications.